A ship’s propulsion system is probably the most important system on a ship, however, this does not negate the purpose of various other systems that support the propulsion system in its operation. A ship will not be able to move without a Propulsion Unit, also known as a Prime Mover, or Propulsion Engine, as part of its propulsion system.

As alluded to, a ship’s propulsion system does not purely consist of just an “Engine”. A standard propulsion system consists mainly of the following:

  • Prime and/or Secondary Propulsion Units/Movers: These engines are usually internal combustion engines, but can also be turbines or electric motors.
  • Damping: Components or systems used to prevent the transmission of vibrations.
  • Flexible Couplings: Couplings used for the connection of shafts between the prime mover and gearbox.
  • Gearboxes: Used to reduce the rpm of the propeller shaft and to allow for the reversal of the shaft.
  • Shafting: Connects the Prime Mover to the propeller for the transmission of power.
  • Propeller: Connected to the shaft, and creates thrust while rotating, driving the ship forward.

There are also various types of Prime Movers usually used in propulsion applications on-board ships. The propulsion types are mostly based on what the Prime Mover uses as fuel. The Prime Mover’s main purpose is to convert the energy of the fuel into the rotation of the shaft and eventually the creation of thrust via the propeller. Primary Prime Mover types use on-board ships are:

  • Diesel Engines (Internal Combustion Compression Ignition Engines). Marine diesel engines are usually classified as slow, medium or high speed.
  • Diesel-Electric: A hybrid system where diesel generators generate electrical power to an electric motor which turns a shaft, or a combination of an electric motor and diesel engine powering a shaft, but the electric motor being used for slow speed loitering on its own, or as additional power at full power.
  • Turbines: Either a gas or steam driven.


Internal Combustion Engine Basics

A diesel engine is considered an Internal Combustion engine, but of the compression ignition type, while a petrol engine, which is also an Internal combustion engine, is of a spark ignition type. A Spark ignition engine achieves combustion by igniting the compressed fuel (petrol) with a spark, while a compression ignition engine ignites the fuel (diesel) by means of the high temperature which a gas achieves when greatly compressed. As large ships mostly operate with diesel engines, the focus of this Learner Guide will be guide towards the use of compression ignition engines.

Internal Combustion engines categorized on whether they run on a 2-stroke cycle or a 4-stroke cycle. One major difference between 2-stroke and 4-stroke IC engines, besides the number of strokes, is the amount of power generated. A 2-Stroke engine has the potential ability to produce twice as much power as a 4-stroke engine of the same size. This is attributed to the fact that in a 2-stroke engine the cylinder fires once per crankshaft revolution, while in a 4-stroke engine the cylinder only fires once for every two revolutions of the crankshaft.

The 2-stroke cycle as shown below occurs as follows:

  • As the piston nears the bottom of its stroke, all the exhaust valves open. Exhaust gases rush out of the cylinder, relieving the pressure.
  • As the piston bottoms out, it uncovers the air intake ports. Pressurized air fills the cylinder, forcing out the remainder of the exhaust gases.
  • The exhaust valves close and the piston starts traveling back upward, re-covering the intake ports and compressing the fresh charge of air. This is the compression stroke.
  • When the piston is at the top of its travel, the cylinder contains a charge of highly compressed air. Diesel fuel is sprayed into the cylinder by the injector and immediately ignites because of the heat and pressure inside the cylinder.
  • The pressure created by the combustion of the fuel drives the piston downward. This is the power stroke.
  • As the piston nears the top of the cylinder, the cycle repeats with step 1.


The 4-stroke cycle as shown below occurs as follows:

  • The intake stroke begins at top dead center of the piston, and as the piston moves down, the intake valve opens. The downward movement of the piston creates a vacuum in the cylinder, causing air to be drawn through the intake port into the combustion chamber.
  • As the piston reaches bottom dead center, the intake valve closes.
  • The compression stroke begins with the piston at bottom dead center and rising up to compress the air. Since both the intake and exhaust valves are closed, there is no escape for the air, and it is compressed to a fraction of its original volume. When the piston is at the top of its travel, the cylinder contains a charge of highly compressed air. Diesel fuel is sprayed into the cylinder by the injector and immediately ignites because of the heat and pressure inside the cylinder.
  • The power stroke begins when the fuel and air mixture is ignited, burns and expands, forcing the piston down.  The valves remain closed so that all the force is exerted on the piston. The power stroke ends as the piston reaches bottom deadcenter.
  • The exhaust stroke begins when the piston nears the end of the power stroke and the exhaust valve is opened. As the piston moves upward towards top dead center, it pushes the burnt gases, resulting from the ignition of the fuel and air mixture, out of the combustion chamber and through the exhaust port. As the piston reaches top dead center, ending the exhaust stroke, the exhaust valve closes, and the intake valve opens to begin the intake stroke for the next cycle.


Marine Diesel Engines

Marine diesel engines are categorized in terms of their rotational speed. Based on the ideal crankshaft rpm of a marine diesel engine, these engines are usually classified as being either of a slow, medium or high-speed type.

Slow Speed Marine Diesel Engines

A Slow Speed Marine Diesel Engine usually has crankshaft rpm of between 100 to 120 rpm and operates on a simple 2-stroke cycle. By using a Slow Speed engine, a direct coupling can be made between the engine and the propeller shaft due to its slow speed in terms of crankshaft rpm. Most merchant vessels, for example oil tankers, cargo ships and bulk carriers, make use of this type of marine diesel engines.

These engines can also burn low grade fuel known as Heavy Fuel Oil (HFO). Due to the high viscosity of HFO, high viscosity being a measure of a fluid’s resistance to flow, the HFO requires pre-heater due to thin it out before being injected into the cylinders. Slow speed engines also have a high thermal efficiency.

This type of engine, due to its simple construction, has less moving parts, thus allowing these engines to have a much higher Mean Time Between Overhaul’s (MTBO) and less overall maintenance tasks to be carried out.  However, with the simple construction comes an increase in weight, making these slow speed marine diesel engines extremely heavy compared to its medium and high-speed counterparts.

Refer to Table for an example of the technical specification of a typical large slow speed marine diesel engine. In this case, the specification data was taken from a Wartsila X92 slow speed engine as shown in the figure as part of the table.

Medium Speed Marine Diesel Engines

A Medium Speed Marine Diesel Engine usually has crankshaft rpm of between 250 to 800 rpm and operates mostly on 4-stroke cycle. By using a Medium Speed engine, a reduction gearbox is required between the engine and the propeller shaft due to its higher speed in terms of crankshaft rpm.

These engines burn either Marine Diesel Oil (MDO) or Marine Gas Oil (MGO). The most notable difference in MDO versus MGO is the method in which it is created. MGO is made from a pure distillate only, while MDO is a blend of different gas oils with a lower viscosity than that of MGO. MDO and MGO has a sulphur content of between 0.3 to 1.5 m/m % (mass per mass percentage).

In comparison to Slow Speed engines, Medium Speed engines have a much better power to weigh ratio, meaning that the amount of power generated per unit mass is more than that of a Slow Speed engine. To understand the technical difference between Slow Speed and Medium Speed Marine Diesel Engines, refer to technical specifications of the MAN 18V48/60CR Medium Speed Marine Diesel Engine below.

High-Speed Marine Diesel Engines

A High-Speed Marine Diesel Engine usually has crankshaft rpm of more than 1000 rpm and operates only on 4-stroke cycle. By using a High-Speed engine, a reduction gearbox is required between the engine and the propeller shaft due to its higher speed in terms of crankshaft rpm. In addition, High-Speed engines are considerably more complex in its construction and operation than that of both Slow and Medium Speed engines.

These engines mostly run on automotive diesel, but can also burn MDO or MGO with low sulphur content, at the expensive having shorter maintenance periods. Automotive diesel has a much lower sulphur content than that of MDO and MGO, but comes at a much higher cost.

In comparison to Medium and Slow Speed engines, High-Speed engines have a much better power to weight ratio than both. To understand the technical difference between Medium Speed and High-Speed Marine Diesel Engines, refer to technical specifications of the MTU 20V8000M71L High-Speed Marine Diesel Engine in the table below.

High-speed marine diesel engine
Technical specifications of high-speed marine diesel engine

Diesel-Electric Propulsion

Diesel-electric propulsion is a combination of diesel engines, electrical generators, and electric motors. The energy created by the fuel burnt in the diesel engine rotates the crankshaft.  This shaft rotation is converted by the electric generator into electrical energy, which can either be directly fed via transformers and voltage regulators to the electric motors or to the storage batteries. The electric motors may either act as a Prime Mover or as a loitering drive for slow-speed maneuvering.

The use of Diesel-Electric Propulsion reduces the ship’s fuel usage as well as the ship’s environmental impact in terms of the emissions associated with constantly running big propulsion diesel engines. Electric motors also greatly minimize maintenance cost since it only consists of one moving part, the rotor, and reduces the number of running hours of the diesel engines as compared to a ship using only diesel engines as its primary mode of propulsion. In addition, electric motors allow for better maneuvering and steering of the ship at especially slower speeds.

The major disadvantages of the use of electric motors as a method of propulsion is as follows:

  • A high risk of electrocution.
  • Higher acquisition cost due to the costs of energy storage.
  • Higher weight contribution to the total ship weight when compared with conventional propulsion. The increase in weight is mostly attributed to the additional support equipment, cabling network and storage batteries required to support the system.


An example of a diesel-electric propulsion system with two electric motors in series acting as the prime mover of the vessel is shown below. In this layout three diesel generators generate enough electrical power supply, which is distributed and controlled via the switchboards, and then routed to the electrical motors. These motors are then coupled to the propeller shaft via a reduction gearbox.

Example of a diesel-electric propulsion system

Gas and Steam Turbines

Turbines are also used as a prime mover in certain ship types, but mostly in naval ships where diesel engines are operated during cruising and the turbines are started when the ship’s maximum speed is required for intercepting a vessel of interest, for example.

The general types of turbines used, are either gas or steam turbines. 

Gas turbines operate on the same principle as turbines used on commercial aircraft, with jet fuel (e.g. Jet A1) as its fuel. The rpm of the turbine shaft is reduced via a reduction gearbox, and ultimately rotates the ship’s propeller or waterjet impeller.  An example diagram of such a gas turbine and its operation is shown below.

Diagram of a Gas Turbine Ship Propulsion System

Steam turbines operate by running high-pressure steam through its turbine blades, which in turn turns the propeller shaft.  The fuel source for a big steam turbine is usually a nuclear power plant. The nuclear reaction in the reactor core provides heat to the steam generator. Steam turbines are usually reserved for big ships like naval aircraft carriers and large destroyers. A diagrammatic example of a nuclear propulsion plant of a ship is shown below.

diagrammatic example of a nuclear propulsion plant of a ship

Combined Prime Movers

More possibilities exist in terms of the various combinations of prime mover types that can be used together to propel a ship. This combination is dependent on the client’s operational requirements for the vessel. Most widely used prime mover combinations include the use of either main propulsion diesel engines with electric motors or main propulsion diesel engines with a gas turbine/s.

More intricate combinations do however exist, as is depicted in the propulsion system layout shown below.

Layout of Combined Prime Mover Types

The above system consists of two diesel propulsion engines, one gas turbine, and two electric loitering drives connected via an intricate combination of gearboxes to drive two propeller shafts.

Legislation Governing Engine Emissions

There are various types of prime mover types and manufacturers to select from within the ship building industry, but how do these prime movers and manufacturers measure up in terms of the “Green” objective discussed mentioned earlier in this Learner Guide, as well as the minimization of air pollution as stipulated by the IMO in the MARPOL regulation Annex VI?

MARPOL Annex VI, Regulations for the Prevention of Air Pollution from Ships addresses the following key points:

  • This regulation sets limits on NOx (Nitrogen Oxides) & Sox (Sulphur Oxides) emissions from ship exhausts, as well as fuel quality requirements.
  • The regulation prohibits the deliberate emission of ozone-depleting substances.
  • The regulation is referred to as Tier I (1997), Tier II (2008), and Tier III (2016).
  • All engine manufacturers must comply with the regulation in terms of the latest adopted Tier of the regulation. Tier III was adopted in January 2016. This is also why you will see marine diesel engine manufacturers advertising their engines as being Tier III compliant.  Most engine manufacturers will allow for even a greater improvement in emissions if the client has enough funding to install additional catalytic converters on the engine exhaust system.
  • The regulation specifies more stringent requirements for certain ECA’s. An ECA is an Emission Control Area. These are special areas that are specifically sensitive to air pollution.


The table below below provides a well-detailed summary regarding the various exhaust gas components, why these components are considered harmful, the primary solutions to reduce these exhaust components and emissions, additional secondary solutions for the reduction of these components off of the engine, as well as alterations that can be made to the fuel solution used to reduce these harmful emissions.

Layout of Combined Prime Mover Types

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